Calypso Actually. The Shapeshifter Series, #4

Titus lives a life of luxury and indulgence as the crown prince of the merpeople. But his "indulgences" have gotten him into a bit of hot water. Having slept with countless human women, he's also produced countless children. Which means he has to pay child support for countless children. Which means the royal treasury is being drained by the baby daddy. When Titus has to go on land for a meeting of leaders, his father decides safeguards must be put into place.
After all, the treasury can't handle Titus fathering another child.That's where Calypso comes in. Having known Titus since childhood, Calypso is familiar with not only him, but his habits. Which makes her the perfect candidate to be his guardian while he's on land. When Titus hears this, he's mildly insulted that his father doesn't trust him. But he's more worried than anything. Having to be alone with Calypso for so long meant he might have to face his feelings.feelings he's kept hidden since childhood.
Until he sees Calypso return his feelings..